The immune system is a closed network of interacting elements and the expression of immunocompetence involves cellular interactions among complementary (idiotypic and anti-idiotypic) cells. In this application I discuss why regulatory cellular interactions between idiotypic and anti-idiotypic cells can best be studied in a well defined antigenic system. Phosphorylcholine antigens induce both in vivo and in vitro idiotypically homogeneous humoral responses. I propose experimental approaches to study the operations of idiotypic and antiidiotypic T cells and their interactions with macrophages in the phosphorylcholine system. Although these approaches include in vivo experiments they emphasize an in vitro approach at the colonal level precisely to quantitate the effects of complementary T cells on idiotypic expression by B cells.